The present invention relates to a hydrodynamic pressure bearing for supporting a rotating member by hydrodynamic pressure which is generated in lubricating fluid filled in a gap defined between the shaft and the sleeve member, and more to a hydrodynamic pressure bearing for use in a motor, including but not limited to, a spindle motor for driving at least one hard disk and more specifically, a DC motor for driving a CD-ROM.
In general, a hydrodynamic fluid pressure bearing includes a shaft, a disk-shaped thrust plate which extends radially outwardly extending from the shaft, a sleeve member which surrounds the shaft and the thrust plate with a gap to rotate relative to the shaft, and lubricating fluid filled in the gap defined by the shaft and the sleeve member therebetween. A radial bearing portion has a plurality of grooves for generating hydrodynamic pressure in the lubricating fluid. The grooves are formed on an outer surface of the shaft and/or the inner surface of the sleeve member at a portion radially opposing the outer surface of the shaft. A thrust bearing portion has a plurality of grooves for generating hydrodynamic pressure in the lubricating fluid. The thrust bearing grooves are formed on the upper and lower surfaces of the thrust plate and/or the inner surface of the sleeve member at a portion axially opposing the thrust plate. The lubricating fluid continuously fills the gap defined by the shaft and the sleeve member so as to hold the radial bearing portion and the thrust bearing portion.
Such hydrodynamic pressure bearing looses its effectiveness if the lubricating fluid decreases substantially at either the radial bearing portion or the thrust bearing portion. In a conventional hydrodynamic pressure bearing, it is liable that lubricating fluid escapes or leaks away from the bearing portion for the following reasons: (1) evaporation; (2) leakage due to thermal expansion of air bubbles in the lubricating fluid (such air bubbles are likely to enter or grow in the lubricating fluid when the lubricating fluid is filled in the gap) with the rising of temperature in the bearing; (3) spattering and/or leakage of the lubricating fluid due to imbalance of the lubricating fluid because of nonuniformity in the shape of the grooves formed at the radial bearing portion and/or the thrust bearing portion; (4) leakage of the fluid due to narrowing of the gap between the shaft and the sleeve member by the thermal expansion of the members caused by temperature rise in the bearing; and (5) leakage due to centrifugal forces generated by the relative rotation of the shaft and the sleeve members, and also due to oil migration phenomena (in which the lubricating fluid spreads along the surface of the shaft and/or the surface of the sleeve member and oozes out of the bearing). These factors tend to decrease the amount of lubricating fluid within the bearing portion and decrease the operating life of the hydrodynamic pressure bearing.